The darters, anhingas or snakebirds are mainly tropical water birds in the family Anhingidae, which contains a single genus, Anhinga.

There are four living species, three of which are very common and widely spread while the fourth is rarer and classified as near threatened by the International Union For Conservation of Nature.

The term snakebird is usually used without any additions to signify whichever of the completely allopatric species occurs in any one region.

It refers to their long neck which has a snake-like appearance when they swim with their bodies submerged, or when mated pairs twist it during their bonding displays.

Darter is used with a geographical term when referring to a particular species. 

Physical description of the Darters

Darters (Anhingidae) are large birds with sexually dimorphic plumage (a layer of feathers that cover the bird and pattern).

They measure about 80 to 100cm in length, with a wing span around 120cm, and weigh some 1,050 to 1,350 grams.

Males have black and dark brown plumage, a short erectile crest on the neck and under parts and are a bit larger overall.

Both have grey stippling on long scapulars and upper wing converts.

The sharply pointed bill has serrated edges, a demognathous palate and no external nostrils.

The darter has completely webbed feet and their legs are short and set far back on the body.  Note: A webbed foot; Is a specialized limb present in a variety of vertebrates that aid in locomotion.

There is no eclipse plumage, but the bare parts vary in colour around the year.

During breeding, however, their small gular sac changes from pink or yellow to black, and the bare facial skin, otherwise Yellow or yellow-green, turns turquoise (blue/green).

The Iris changes in colour between Yellow, red or brown seasonally.

The young hatch naked, but soon grow white or tan down.

Darter vocalizations include a clicking or rattling when flying or perching. In the nesting colonies adults communicate with croaks, grunts or rattles.

During breeding, adults sometimes give a caw or sighing together with hissing calls. 

Distribution

Darters are mostly tropical in distribution, ranging into subtropical and barely into warm temperate regions.

They tropically inhabit fresh water lakes, rivers, marshes, swamps and are less often found along the sea shore in brackish estuaries, bays, lagoons and mangrove.

Most are sedentary and don’t migrate. Their preferred mode of flight is soaring and gliding, in flapping flight they are rather cumbersome.

On dry land, darters walk with a high stepped gait, wings often spread for balance, just like pelicans do.

They do gather in flocks, sometimes up to about 100 birds and frequently associate with storks, herons or Ibises, but are highly territorial on the nest.

Diet

Darters feed mainly on mid-sized fish, far more rarely, they eat other aquatic vertebrates and large invertebrates of comparable size.

These birds are foot-propelled divers which quietly stalk ambush their prey.After the ambush, they use their sharply pointed bill to impale the food animal.

They do not dive deep but make use of their low buoyancy (the ability of something floating on water)which is made possible by wettable plumage, small air sacs and denser bones.

After they have stabbed the prey, they return to the surface where they toss their food into the air and catch it again so that they can swallow first.

Darters like cormorants, they have a vestigial preen gland and their plumage gets wet during diving.

To dry their feathers after diving, darters move to safe location and spread their wings. 

Breeding  and Reproduction process of the Darter

They usually breed in colonies, occasionally mixed with cormorants or herons. They pair bond monogamously at least for a breeding season.

There are many different types of displays used in mating;

Males display to attract females by raising their wings to wave them in alternating fashion, bowing and snapping the bill or giving twigs to potential mates.

To strengthen the pair bond, partners rub their bills or wave, point upwards or bow their necks in unison.

Breeding is seasonal peaking in March or April at the northern end of their ranges, elsewhere they can be found breeding all year round.